1. Field of the Invention
The present invention relates to a method for manufacturing silicon thin-film solar cells, more particularly, to a method for manufacturing silicon thin-film solar cells with graded bandgaps.
2. Description of Related Art
Due to the finite nature of conventional energy such as petroleum and coal, alternative forms of energy have been developed to replace the conventional energy. Among all the forms of alternative energy, solar energy is abundant and environmentally friendly, so research about solar cells has been vigorously pursued. The solar cell is a photovoltaic device to convert light into electricity, and the structurally simplest solar cell is a single junction solar cell. As shown in FIG. 1, a single junction solar cell includes: a substrate 11, a first electrode 12, a p-type semiconductor layer 13, an intrinsic layer 14, an n-type semiconductor layer 15, and a second electrode 16. Accordingly, when sunlight passes through the substrate 11 and the first electrode 12, the PIN junction would absorb light and thus separates electrons and holes. Subsequently, under the intrinsic electric field, the obtained electrons and holes would move to the n-type and p-type semiconductor layers respectively to contribute current. Finally, the current is derived out through electrodes to form electricity for usage or storage.
Silicon solar cells are the major commercially used solar cells. Among them, amorphous silicon solar cells can be fabricated in lower cost, but exhibit lower photoelectric conversion efficiency than monocrystalline or polycrystalline silicon solar cells and have the drawback of photodegradation. In order to resolve the problems, microcrystalline silicon solar cells with inhibited photodegradation have been developed.
Additionally, in order to achieve broadband absorption and improved photoelectric conversion efficiency, tandem solar cells have been suggested, such as double junction and triple junction solar cells. FIG. 2 shows a cross-sectional view of a conventional double junction silicon solar cell. As shown in FIG. 2, in the double-junction silicon solar cell, the second microcrystalline silicon PIN junction unit (including a second p-type semiconductor layer 23′, a second intrinsic layer 24′ and a second n-type semiconductor layer 25′) is formed on the first amorphous silicon PIN junction unit (including a first p-type semiconductor layer 23, a first intrinsic layer 24 and a first n-type semiconductor layer 25) on the first electrode 22. Accordingly, the first amorphous silicon PIN junction unit can absorb light of 350 nm to 800 nm, while the second microcrystalline silicon PIN junction unit can absorb light of 350 nm to 1200 nm, resulting in broadband absorption and enhanced photoelectric conversion efficiency. Moreover, in order to further enhance the photoelectric conversion efficiency, an intermediate layer 27 may be formed between the first amorphous silicon PIN junction unit and the second microcrystalline silicon PIN junction unit to increase the reflection of light.
Although broadband absorption can be achieved by stacking PIN junction units of different bandgaps, the choice of material used in the tandem solar cells is limited in consideration of current matching, and the method for fabricating the tandem solar cells is more complex. Additionally, the manufacture of the intermediate layer for the enhancement of the photoelectric conversion efficiency is difficult and causes an increase in manufacturing cost.